Reduced PTH pad for enabling core routing and substrate layer count reduction

ABSTRACT

Embodiments are directed to semiconductor packaging having reduced sized plated through hole (PTH) pads by eliminating the margin of the pad-to-PTH alignment and enabling finer traces on the core of the substrate.

RELATED APPLICATIONS

The present application is a Continuation of U.S. application Ser. No.14/097,932 filed Dec. 5, 2013, which is a Divisional of U.S. applicationSer. No. 12/973,596 filed Dec. 20, 2010, both entitled “REDUCED PTH PADFOR ENABLING CORE ROUTING AND SUBSTRATE LAYER COUNT REDUCTION”.

FIELD OF THE INVENTION

Embodiments of the present invention are directed to semiconductorpackaging and, more particularly, to reduced sized plated through hole(PTH) pads by eliminating the margin of the pad-to-PTH alignment andenabling finer traces on the core of the substrate.

BACKGROUND INFORMATION

A printed circuit board (“PCB”) is a multilayer board that includesprinted circuits on one or more layers of insulative (a.k.a. dielectric)material. A printed circuit is a pattern of conductors that correspondsto the wiring of an electronic circuit formed on one or more layers ofinsulative material. The printed circuit board includes electricaltraces that are routed on the various layers of the PCB. PCBs alsoinclude vias which are solid electrical paths connecting one layer toanother layer. A via can be used to connect a trace on one layer of aPCB to another trace on another layer of the PCB.

A PCB also includes other layers of metallization for ground planes,power planes or reference voltage planes. In many instances a signalcarrying via must be routed through one or more of these planes. Thesignal carrying via cannot electrically connect or couple to theseplanes. If the signal carrying via does couple or connect to one ofthese planes, the integrity of the electrical circuit is compromised. Asa result, anti-pads or plane clearances are required to separate signalcarrying vias from ground planes, power planes, or planes having areference voltage. An anti-pad is a plane clearance. Generally, aminimum anti-pad clearance is specified in the design after balancingfactors that tend to minimize the anti-pad size and those factors thattend to maximize anti-pad size. The anti-pads would be minimized toreduce noise by closely shielding adjacent pins with reference planes,to reduce electromagnetic interference (EMI) by minimizing aperturesizes in reference planes, and to maintain a strong reference to groundfor single-ended signals and ground referenced differential signals. Theanti-pads would be maximized to maximize voltage breakdown spacingbetween the pin and the reference plane, to increase manufacturabilityby reducing the chance of shorting, and reduce reflection in a highspeed gigabit serial system by reducing the capacitive effect of aplated through hole (used instead of a via).

The semiconductor industry has seen tremendous advances in technology inrecent years that have permitted dramatic increases in circuit densityand complexity, and equally dramatic decreases in power consumption andpackage sizes. Present semiconductor technology now permits single-chipmicroprocessors with many millions of transistors, operating at speedsof tens (or even hundreds) of MIPS (millions of instructions persecond), to be packaged in relatively small, air-cooled semiconductordevice packages. A by-product of such high density and highfunctionality in semiconductor devices is an ever increasing pressure toproduce PCBs having higher density designs. With increasingly higherdensity designs, the risk becomes greater that the established industryreliability specification for minimum dielectric spacing between holewall and adjacent conductive features will be violated. There is also apossibility that with increased device density, the industry will lowerthe minimum dielectric spacing between features.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and a better understanding of the present invention maybecome apparent from the following detailed description of arrangementsand example embodiments and the claims when read in connection with theaccompanying drawings, all forming a part of the disclosure of thisinvention. While the foregoing and following written and illustrateddisclosure focuses on disclosing arrangements and example embodiments ofthe invention, it should be clearly understood that the same is by wayof illustration and example only and the invention is not limitedthereto.

FIG. 1 is a cross-sectional view of a multi-layer semiconductor packageaccording to one embodiment of the invention;

FIGS. 2A-2E are cross-sectional views illustrating the process formaking plated through holes (PTHs) in a core according to one embodimentof the invention;

FIG. 3 is a cross-sectional view of a reduced pad substrate according toone embodiment of the invention; and

FIGS. 4A and 4B compares current PTH pads shown from a top view of thepackage 1F layer (only front side of the layer marked as 212 in FIG. 2E)with PTH pad, antipad, routing traces (also referred as fine line andspace—FLS), and PTH pitch, according to embodiments of the invention.

DETAILED DESCRIPTION

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearances of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

Referring now to FIG. 1, there is shown an illustrative cross-sectionalview of semiconductor package 100. The package 100 may include amulti-layer circuit board comprising a core 102 including one or moredielectric layers 104 built-up on either side of the core 102. Aplurality of conductive vias 106 may be provided to create electricallyconductive paths through the dielectric layers 104. Plated through holes(PTHs) 108 may be provided to provide vertical connections through thecore 102 to conductive layers 110. A semiconductor die 112 may beflip-chip connected to external conductive layers with solder balls 114.

Example materials for the core 102 may comprise ceramic or glassdielectrics. For example, a core 102 may comprise one or more selectedfrom a group that comprises alumina, zirconia, carbides, nitrides, fusedsilica, quartz, sapphire, or any other ceramic or glass dielectricmaterials. In one embodiment, the ceramic materials for the core 102 mayhave a full density or an amount of porosity. In another embodiment, thematerials for the core 10 may have a Young's modulus that may be higherthan 20 GPa (e.g., at a room temperature). For example, the materialsfor the core 102 may have a Young's modulus that may be higher than 100GPa (e.g., at a room temperature). In another embodiment, the materialsfor the core 102 may have a coefficient of thermal expansion (CTE) thatmay be in proximity to that of a semiconductor die to be coupled to thecore 102. For example, the core 102 may comprise materials that may havea CTE lower than 12 ppm/° C. In one embodiment, the ceramic core 102 mayintegrate high-k ceramic thin film decoupling capacitors.

In yet another embodiment, the ceramic materials for the core 102 maycomprise alumina that may be compounded with silica or other elements.In another embodiment, the ceramic materials may be compounded with,e.g., around 50% to 100% Al₂O₃. In another embodiment, a thickness ofthe core 102 may be determined by a Young's modulus and a stiffness ofthe core 102. In one example, a stiffness of the core 102 may beproportional to Ed³, wherein E represents the Young's modulus and drepresents the thickness. In one embodiment, the core 102 may have athickness that may be from around 50 um to around 400 um; however, insome embodiments, the core 102 may have a different thickness. Inanother embodiment, the materials for the core 102 may have a thermalconductivity that may be from around 2 W/m·k to around 50 W/m·k. Inanother embodiment, the materials may have a dielectric strength fromabout 9 KV/mm to around 50 KV/mm. However, in some embodiments, othermaterials that have a different thermal conductivity and/or differentdielectric strength may be utilized.

In one embodiment, the materials may have a dissipation factor lowerthan 0.01 (e.g., at 1 GHz). For example, the materials may have adissipation factor lower than around 0.0003. In yet another embodiment,the materials may have a dielectric constant from e.g., around 5 toaround 20 (e.g., at 1 GHz). In another embodiment, the material may havewater absorption of around zero. However, in some embodiments, otherceramic or glass materials have a different combination of propertiesmay be utilized.

In another embodiment, the core 102 may comprise inorganic materialsthat may have a Young's modulus higher than that of, e.g., polymer-basedorganic core materials. For example, the inorganic materials may have aYoung's modulus that may be 2 to 14 multiples higher than that of thepolymer-based organic core material; however, in some embodiments, theinorganic materials may have a different Young's modulus. In oneembodiment, the core 102 may have a comparable or increased stiffnesswith a reduced core thickness.

The conductive layer 110 may be selectively patterned to provide a firstset of one or more conductive elements such as traces, planes orinterconnects pins on the upper and/or the lower side of the core 102.The dielectric layers 104 may be provided over the core 102. Examplematerials for the dielectric layer 104 may comprise particulate-filledsuch as Ajinomoto build-up film (ABF), or glass fiber reinforced epoxyresin such as prepreg materials, or other insulating or dielectricmaterials. In one embodiment, surface roughening methods and/or adhesionpromotion methods such as silane treatment may be utilized to bond thedielectric layer 104 to the core 102. For example, one or more greensheets for the core 102 may be roughened, e.g., prior to firing, toincrease surface roughness of the core 102.

A set of one or more through holes 108 may be selectively formed in thestructure. In one embodiment, a laser may be used to provide the throughholes 108. The laser may have a pulse width in a magnitude of ananosecond. In some embodiments, the laser may have a pulse width thatmay be shorter than a nanosecond. In one embodiment, the laser may havea spectrum in a range from infrared radiation (IR) to deep ultraviolet(DUV). Examples for the laser may comprise Q-switched or mode-lockedNd:YAG or Nd:YVO4 lasers that may have a harmonic of 1064 nm, 532 nm,355 nm, 266 nm or any other harmonics; Q-switched or mode-locked Nd:YLFlasers that may have a harmonic of 1053 nm, 527 nm, 351 nm, 263 nm orany other harmonics; or fiber laser. In another embodiment, the lasermay have a pulse repetition frequency in a level from kHz to MHz;however, in some embodiments, any other lasers or means may be used.

Referring to FIGS. 2A-2E, there is shown a process flow using laserdrilling to make plated through hole (PTH) vertical interconnectionsthrough a core 200. In FIG. 2A there is shown a core 200 having a resinrich outer area 202. Resin-rich here implies to the zone of thesubstrate core from where the glass fibers and filler materials, used toprovide desirable mechanical properties (such as high stiffness or lowerCTE), are preferentially pulled into the inner side of the core of thesubstrate, leaving top and bottom most (˜10-20 μm) approximately 90-100%organic epoxy resin. In FIG. 2B a through hole 208 may be laser drilledand have a diameter in a ranges from approximately 100 μm in the top andbottom side, and 60-90 μm at the center of the through hole (TH), makingthe effective TH look like an hour glass structure. In some embodiments,a through hole (TH) 208 may have a different size. In anotherembodiment, one or more redundant through holes (not shown) may bedrilled in the structure. A copper (Cu) foil strip 204 may be layed downand thereafter a dry film resist (DFR) process utilized to create apattern.

In FIG. 2C, the DFR may be exposed and then stripped to form finer lineand space (FLS) pattering of (<20 μm wide) 210. In FIG. 2D the throughholes 208 may be filled using, for example, electroless and thenelectrolytic copper plating to fill the vertical interconnections 210.In other embodiments, other conductive materials may be used. The coppermay also fill other pattern voids to create conductive areas 212 aswell.

As shown in FIG. 2E, additional dielectric layers 214, such as,Ajinomoto build-up film (ABF), to begin the build-up process.

Referring again to FIGS. 2A through 2E, there is shown the cross sectionof the substrate panel during the sequential process steps to enable asubstrate with reduced pad size via semi-additive process (SAP) on thecore of the substrate. FIG. 2A depicts the incoming prepreg core 200with resin rich area 202 just below the copper foil which is claddedinto the incoming core material. FIG. 2B shows substrate panel withcopper foil stripped down to approximately 1-2 μm thick by etching, andthen the through hole (TH) via is drilled through laser drilling 208,followed by electroless copper plating 206 and then one dry film resistis applied for subsequent patterning 204. Then FIG. 2C depicts the crosssection of the exposed, developed and then stripped of the DFR materialto form the trenches 210 for the routing traces. FIG. 2D depicts thecross sectional view of the substrate that is filled with electrolyticcopper filling. This filling fills up the TH vias 210, traces 212 andthe via pads 216 simultaneously. Then FIG. 2E shows the cross sectionalview with DFR stripped completely, then dielectric material laminated onthe front and back side of the substrate process 214. Then subsequent BUprocess may be continued.

FIG. 3 illustrates a cross sectional view of a reduced pad sizedsubstrate according to one embodiment of the invention. The substratemay comprise a core 200 with a resin-rich outer layer 202 having one ormore ABF build-up layers 214 on both side of the core 200. A pluralityof plated through holes (PTHs) 210 may provide vertical electrical pathsthrough the core 200 and connect to vias 206 and other conductive areassuch as traces 212 in the various ABF layers 214. Solder balls 300 mayalso be provided on the upper side of the substrate, such as for flipchip connections to a semiconductor die as well as solder balls on thebottom side of the substrate.

Referring to FIGS. 4A and 4B, according to embodiments of the invention,the laser drill size for creating the through holes in the core 200 maybe approximately 100 um. This is substantially smaller than currentlyused drill sizes which may be upwards of 250 um. The usage of the laserdrilling process may allow for pad sizes as small as 100 μm to 150 μm oreven smaller (216 in FIG. 2D). Additionally, this allows for corerouting (RTG) and line spacing (L/S) of 20 μm/20 μm due to theapplication of semi-subtractive process with copper foils in top resinrich core, or application of semi-additive process (SAP) in the core asshown through FIG. 2A-2E. With this process, the anti-pad size may be assmall as 200 μm which allows for core referencing which is not possiblein prior art designs.

Smaller pads allow more number of lines in between the pads (as shown inFIG. 4B). In addition, smaller pads can drive smaller anti-pad, so thatthe core layer can be used as a reference layer. Both of theseadvantages provide a package layer count reduction. In addition to theseadvantages the smaller pad is enabled by smaller drill size, by laserdrilling to make the cost avoidance of mechanical drill, which can befilled by Cu plating making it more robust for package inductor basedpower delivery.

Reduced PTH pad and reduced trace width allows more numbers of traces toescape between the pads, reduces antipad size and reduces the PTH pitch.All these are beneficial for substrate package design. While, morenumber of traces allows more routing lines in the same package realestate making the package effective for higher input/output (IO) countat the same real estate, tighter PTH pitch makes the package size (formfactor) effectively smaller.

Comparing FIGS. 4A and 4B, FIG. 4B shows the various traces and numberof routing wires that can now pass between the pads as a function of padsize according to embodiments. Increased number of traces allows moresignal can be confined in the layer 1F (212—in FIG. 2E). If thisincrease in signal count is adequate it is possible to eliminate a layerpair from the BU layer.

The above description of illustrated embodiments of the invention,including what is described in the Abstract, is not intended to beexhaustive or to limit the invention to the precise forms disclosed.While specific embodiments of, and examples for, the invention aredescribed herein for illustrative purposes, various equivalentmodifications are possible within the scope of the invention, as thoseskilled in the relevant art will recognize.

These modifications can be made to the invention in light of the abovedetailed description. The terms used in the following claims should notbe construed to limit the invention to the specific embodimentsdisclosed in the specification and the claims. Rather, the scope of theinvention is to be determined entirely by the following claims, whichare to be construed in accordance with established doctrines of claiminterpretation.

What is claimed is:
 1. An apparatus, comprising: a substrate compringresin throughout the substrate, fibers, and at least one resin richouter area; a plurality of plated through holes (PTHs) extending in thesubstrate to provide electrical paths through the substrate, the PTHshaving a diameter of approximately 60 to 90 μm at a center of the PTH; aplurality of trace lines between the PTHs wherein the trace lines have awidth of approximately 20 μm and the wherein the trace lines have anedge to edge distance of approximately 20 μm; a conductive viaelectrically connected to a PTH of the plurality of PTHs; a conductivepad electrically connected to the vias; and a semiconductor dieconnected to the conductive pad through conductive structures.
 2. Theapparatus as recited in claim 1 wherein the resin rich outer area isapproximately 10-20 μm thick.
 3. The apparatus as recited in claim 1wherein the resin rich outer area comprises approximately 90-100%organic resin.
 4. The apparatus as recited in claim 1 wherein the fibersare glass fibers.
 5. The apparatus as recited in claim 1 wherein theconductive structures are solder balls.
 6. The apparatus as recited inclaim 1 wherein the substrate comprises a core region and the pluralityof PTHs extend through the core region.
 7. The apparatus as recited inclaim 1 wherein the substrate additionally comprises filler material.